Multi-axial grid or mesh structures with high aspect ratio ribs

ABSTRACT

A multi-axial geogrid possesses a series of interconnected strands or ribs that are arranged along at least two different axes within the plane of the structure. The strands or ribs have an aspect ratio, defined as the ratio of the thickness to width, of greater than 1.0, thickness being the direction normal to the plane of the structure. The geogrid can be manufactured by modifying the process parameters in order to create high aspect ratio ribs, using any of the various known methods for producing geogrids. A reinforced civil engineering structure, and method therefor, is formed by embedding in soil one or more horizontal layers of geogrid having high aspect ratio ribs. The reinforced structure shows improved rutting performance when subjected to vehicular traffic.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/618,625 filed Sep. 14, 2012, issuing as U.S. Pat. No. 9,556,580 onJan. 31, 2017, which is a continuation of U.S. application Ser. No.12/379,058 filed Feb. 11, 2009, which claimed priority from U.S.provisional application Ser. No. 61/064,102, filed Feb. 15, 2008, thepriority of which is hereby claimed.

FIELD OF THE INVENTION

The present invention relates generally to polymeric grids and bondedcomposite mesh structures used for mechanical stabilization or retentionin civil and construction engineering applications. More specifically,the present invention relates to such polymeric grids and composite meshstructures, especially integral geogrids, which have high aspect ratioreinforcing ribs. These grids or mesh structures possess strands or ribsthat are arranged along at least two different axes within the plane ofthe structure.

BACKGROUND OF THE INVENTION

The mechanisms for explaining the reinforcing effect of a geosyntheticthat includes multi-axial geogrids on a granular material such as soilor stone, for example, when a reinforced structure is used to resist therutting effects of vehicle traffic, are still being developed. Studies,such as reported in the article entitled Aggregate Base ResidualStresses Affecting Grid Reinforced Flexible Pavement Response, Kwon etal., appearing in the International Journal of Pavement Engineering, iFirst article 2007, 1-11, have shown that it is not possible to describethe reinforcing effect based on the individual properties of the geogridand granular materials alone. It is therefore necessary to introduce theconcept of a composite matrix or rigid confinement layer consisting ofthe geogrid and the granular material that is confined and restrained bythe geogrid. When a granular civil engineering structure is reinforcedby a geosynthetic material of proper design having high junctionstrength and structural integrity, such as an integral geogrid, theresulting composite matrix is capable of withstanding increased loadsand/or decreased deformations compared to an unreinforced structure.

Some of the factors that can explain the reinforcing effect of thiscomposite matrix include (1) increase in the sustainable load due toreduced sliding and rolling of granular material due to its confinementby the geogrid; (2) increase in the shear resistance of the granularmaterial due to frictional forces between the granular materials and thegeogrid; (3) increase in the resistance to lateral bulging of the massof granular material by the restraining effects of a stiff geogrid withstrong junctions; and (4) increase in the resistance to deformationthrough strong mechanical bonds that form between the granular materialand the rigid geogrid. The mechanical bonding, or interlocking, effectis believed to be dependent on the relative sizes of the geogridaperture compared to the granular material.

Aperture stability modulus (ASM), a relative measure of in-plane geogridrigidity, is one property that has been identified as useful forquantifying the reinforcing effect related to the rutting resistance ofwheel loads from vehicular traffic. Actual field tests to compare thetraffic performance of flexible pavements using multi-axial geogrids asreinforcement for the base courses were conducted with geogrid materialsmade by several methods, i.e. integral geogrids formed by punching andstretching flat sheets, integral geogrids formed from extruded andstretched grids, and geogrids formed from stitch-bonded woven fabrics.See (Webster, Steve L.; Multi-axial geogrid Reinforced Base Courses forFlexible Pavements for Light Aircraft: Test Section Construction,Behavior under Traffic, Laboratory Tests, and Design Criteria; ReportDOT/FAA/RD-92; December 1992. The results demonstrated that the relativeresistance of asphalt pavement test sections to rutting by a heavy wheelload correlated well with a proposed test for aperture stabilitymodulus.

The aperture stability modulus test is carried out by clamping a sampleof a multi-axial geogrid in a fixture to stabilize it and then attachinga small clamp to the ribs in an area around a junction. The junctionclamp is attached to a system of pulleys and weights such that a knowntorque can be applied to the junction. The amount of torque required torotate the area around the junction by a designated number of degrees isdefined as the aperture stability modulus.

If carried out only on one single junction, the aperture stabilitymodulus test can easily distinguish the relatively rigid junctions of anintegral geogrid having integrally-formed junctions from a geogridhaving junctions that are much less rigid when only bonded together. Theaperture stability modulus also provides a relative indication of thestiffness of a geogrid. For example, a geogrid constructed by stitchbonding polymeric filaments will be quite flexible in comparison to anintegral geogrid formed by punching and subsequently orienting polymericsheet.

Because a correlation had been established relating aperture stabilitymodulus to geogrid performance, recent design work for new geogridstructures has sometimes focused on maximizing the aperture stabilitymodulus. In particular, U.S. Pat. No. 7,001,112 (hereinafter the '112patent), owned by the assignee of the present invention, teaches thatthe aperture stability modulus is increased by approximately 65% over aconventional biaxial geogrid, having similar weight in grams per squaremeter, by employing six-rib structures with triangular apertures. Thesubject matter of the '112 patent is expressly incorporated into thisspecification by reference as if the '112 patent were set forth hereinin its entirety.

One means of increasing aperture stability modulus, after ensuring thatthe grid structure possesses rigid connecting junctions or nodes, is toincrease the bending stiffness of the ribs in the plane of the geogrid.The higher the resistance of the ribs to in-plane shear or bendingmoment, the more the ribs will contribute to the apparent “stiffness” ofthe aperture as measured by the techniques employed to determineaperture stability modulus. Optimum in-plane shear and bendingresistance for a given mass of rib material can be obtained by choosinga low aspect ratio rib shape, where aspect ratio (AR) is defined as themost representative value of the ratio of the thickness or height of therib cross-section to the width of the rib cross-section. To maximize theshear and bending resistance, the low aspect ratio rib, typically withAR less than one, has therefore been the preferred rib shape to maximizeaperture stability modulus. The teaching of the '112 patent in factemploys a rib with aspect ratio as low as 0.38 in order to achieve highaperture stability modulus.

Low aspect ratio multi-axial geogrids have been specified in U.S. Pat.Nos. 5,156,495 and 5,419,659. In U.S. Pat. No. 5,156,495, civilengineering structures are disclosed where biaxially-oriented meshstructures have an AR of the oriented strands substantially less thanunity. Finally, in U.S. Pat. No. 5,419,659, methods for constructingcivil engineering structures are disclosed using biaxial geogrids wherethe AR of the thickness to the width of the orientated strands issubstantially less than unity.

Multi-axial geogrids can be manufactured by several different methods,some of which have been used for over 25 years. Generally, such geogridsconsist of ribs or strands made of oriented plastics materials.Multi-axial integral geogrids are manufactured by extruding anintegrally cast sheet of polymer material which is subjected to adefined pattern of holes or depressions followed by the controlledbiaxial orientation of the polymer material to cause the holes ordepressions to form into apertures or mesh openings. The manufacture ofsuch multi-axial geogrids and other integral polymer grid structures canbe accomplished by well known techniques. As described in detail in U.S.Pat. Nos. 4,374,798, 4,590,029, 4,743,486, 4,756,946 and 7,001,112, astarting polymer sheet material is first extruded and then punched toform the requisite defined pattern of holes or depressions. In U.S. Pat.Nos. 3,252,181, 3,317,951, 3,496,965, 4,470,942, 4,808,358 and5,053,264, the starting material with the requisite pattern of holes ordepressions is formed in conjunction with a polymer extrusion. Otherknown methods for manufacturing multi-axial geogrids include, forinstance, stitch bonding fabrics made of filaments, such as polyester,and applying a flexible coating, such as a PVC coating, or by weaving orby knitting, or even spot welding oriented plastic strands together. Itis intended that the present invention be applicable to all types ofmulti-axial geogrids regardless of the method of forming the geogrid.However, integral geogrids are preferred.

Multi-axial geogrids currently in use for civil engineering structureshave aspect ratios less than unity. For example, the aspect ratios ofintegral geogrids (with integral junctions) that are formed by punchingand orienting plastics sheet are less than 1, typically in the range 0.2to 0.9. Stitch-bonded fabric geogrids typically consist of multiple yarnbundles that are bonded by stitching and/or a coating process; themultiple yarn bundles are laid side by side to produce a compositestrand of two or more adjacent yarns. Aspect ratios of these fabricgeogrids typically range from about 0.1 to 0.6. Aspect ratios of gridsproduced by orienting extruded net structures are typically around 0.25to 0.9. By comparison, geogrids formed by spot-welding or otherwisebonding oriented plastics strips often have very low aspect ratios, forexample, less than or equal to approximately 0.2.

A high aperture stability modulus alone, however, does not necessarilyconstitute a multi-axial geogrid that will perform optimally whenincorporated as a reinforcement or retention means for civil engineeringapplications. Geogrids formed by spot-welding or otherwise bonding lowaspect-ratio oriented plastic strands together, for example, possessvery high values of aperture stability modulus but, when incorporatedinto a civil engineering structure, have been demonstrated to havelimited ability to resist the rutting effects of vehicle traffic whencompared to integral geogrids formed with integral junctions.

Also, one can simply increase the thickness of the grid structure toincrease the aperture stability modulus, but this approach adds weightand increases product cost. The preferred geogrid is one that achievesthe highest amount of reinforcing effect with the lowest geogrid weightand cost.

Multi-axial geogrids have generally been formed such that the meshes orapertures have a square or rectangular shape and consist of series ofparallel ribs or strands that intersect at right angles to formjunctions. The ribs or strands are arranged in both a longitudinaldirection, i.e. in the machine direction of the finished product as wellas transversely, i.e. at right angles to the longitudinal strands. Thestrands usually consist of oriented polymer material in order to achievehigh tensile strength with relatively low weight. Such multi-axialgeogrids provide a reinforcing effect by distributing applied stressesto the longitudinal and transverse ribs.

SUMMARY OF THE INVENTION

In light of the foregoing discussion, the present invention is based onthe surprising discovery that a multi-axial geogrid or other meshstructure having ribs or strand cross-sections with a high aspect ratioprovides improved performance when the geogrid is utilized as areinforcing component in a civil engineering structure, for example, ina roadway or airstrip, where one or more layers of geogrid are employedas both subgrade improvement or base course reinforcement. Thisdiscovery reveals that junction stability alone, as measured by theaperture stability modulus, does not fully describe the ability of amulti-axial geogrid in a reinforced structure to resist the ruttingeffects of vehicle traffic. While not intending to be bound hereby, itis presently believed that a high aspect ratio rib provides increasedconfinement or interlocking of the granular material contributing to theenhanced composite matrix or rigid confinement layer effect, and thisenhanced effect results in a civil engineering structure that betterresists the applied vehicle load.

Another aspect of the present invention is the combination of a highaspect ratio rib with an improved load distribution geogrid geometry.For example, a multi-axial geogrid possessing six strands radiating fromeach junction and thus having triangular apertures will more effectivelydistribute applied stresses compared to multi-axial geogrids havingrectangular apertures. When a six-strand geogrid is formed with highaspect ratio ribs as demonstrated by the instant invention, theresistance of the geogrid to rutting by vehicle traffic is greatlyenhanced compared to similar geogrids with low aspect ratio ribs.Triangular geogrids formed with substantially equilateral triangularopening or apertures are preferred.

Moreover, the magnitude of improved resistance to rutting in going froma low to high rib aspect ratio is found to be greater for triangulargeogrids compared to the magnitude of improvement obtained withrectangular geogrids. In addition to the increased confinement of thegranular material provided by the high aspect ratio ribs, the triangularaperture arrangement provides a superior configuration of reinforcingribs that more effectively distributes applied stresses within thegeogrid structure. Hence, a higher ASM value may be desirable forrectangular geogrids, whereas the ASM value does not appear to be afactor in triangular geogrids. In any event, it is intended that thepresent invention be applicable to all multi-axial geogrids and meshstructures, regardless of whether the apertures are triangular orrectangular in shape.

Furthermore, it is widely accepted that the investigation of themechanisms that affect the performance of geosynthetics, and inparticular integral geogrids, in granular layers under rolling wheeltrafficking conditions is a complicated issue. The generation of a‘stabilized layer’ within the unbound aggregate is the ultimate goal andin order to achieve this goal the unbound aggregate is required to beconfined or interlocked and its movement restricted in order to reducethe vertical deformation due to loading. It is this vertical deformationthat produces the detrimental, common rutting channels seen on manyroads and hard surfaces. The most critical region in providingstabilization is the geogrid/aggregate interface as this interface isthe primary region that will dictate the degree to which the geogridinterlocks with the aggregate and thus restricts the movement of theaggregate, Hence, the geogrid/aggregate interface can have a majoreffect on the performance of the stabilized layer.

Historically, biaxially oriented geogrids have been produced with simplerectangular or square shaped apertures with all the tensile elements orribs forming an angle of 90° at the point of their junction orintersection. Although some thought has been applied to the optimum sizeof the aperture in relation to the size of the intended aggregate in aparticular application, the aperture shape and intersection angle haveremained the same until recently.

As discussed above, an integral geogrid having triangular apertures inaccordance with the '112 patent has been shown to be a more efficientuse of the available polymer and to reduce the trafficking deformationin various applications when compared to integral geogrids withrectangular or square apertures. In combination with high aspect ratioribs in accordance with the present invention, it is believed that theshape of the aperture may also be a factor in the improvements observed.As equilateral triangular apertures have an included angle of only 60°between the ribs instead of the previous 90° rectangular or squareapertures, the “wedge action” of this smaller angle may provide agreater, and possibly faster, confinement for the aggregate,particularly for the aggregate in contact with the geogrid but alsoextending upwards from this area towards the surface. Full scaletrafficking testing has shown that after the test is complete theexcavation and removal of the aggregate from the geogrid surface issignificantly more difficult for equilateral triangular apertures thanfor rectangular or square apertures as the aggregate appears to betightly wedged into the 60° angles of the triangular apertures.

The aspect ratio for the tall or high AR ribs for multi-axial geogridsin accordance with the present invention should be greater than 1.0. Itis currently believed that the aspect ratio for triangular geogrids inaccordance with the present invention should preferably be in the rangebetween about 1.4 and about 2.2, but can be as low as greater than 1.0and as high as about 2.5, or above. It is currently believed that theaspect ratio for rectangular geogrids in accordance with the presentinvention should be in the range between greater than 1.0 and about 4.0,with an accompanying aperture stability modulus greater than about 0.3Nm/degree at 20 kg-cm of applied torque and, more preferably, greaterthan about 0.45 Nm/degree at 20 kg-cm applied torque. These preferred ARdimensions and ASM values may vary depending upon the differentstructures and methods for making different type multi-axial geogrids,and variations in the dimensions can be developed through testing bythose skilled in the art. Further, the cross-section of the high AR ribsaccording to the present invention need not be exactly rectangular, forexample, may have concave sides.

Accordingly, it is an object of the present invention to provide amulti-axial grid or mesh structure having a generally uniform array ofsubstantially straight oriented transverse strands or ribsinterconnected by junctions extending transversely across the grid ormesh structure in spaced apart rows and a plurality of substantiallystraight oriented connecting strands or ribs interconnecting thejunctions in adjacent rows to form apertures or openings betweenadjacent oriented strands or ribs and junctions, in which the strands orribs have an aspect ratio greater than 1.0.

Another object of the present invention is to provide a grid or meshstructure in accordance with the preceding object in which the grid ormesh structure is in the form of a triangular geogrid and the aspectratio of the oriented strands or ribs is between greater than 1.0 andabout 2.5, and preferably between about 1.4 and about 2.2, and theapertures or openings are preferably in the shape of an equilateraltriangle.

A further object of the present invention is to provide a grid or meshstructure, in accordance with the first object in which the grid or meshstructure is in the form of a rectangular geogrid and the aspect ratioof the oriented strands or ribs is between greater than 1.0 and about4.0.

Still another object of the present invention is to provide a grid ormesh structure in accordance with the preceding object in which the gridor mesh structure also has an aperture stability modulus greater thanabout 0.3 Nm/degree at 20 kg-cm of applied torque, and preferablygreater than about 0.45 Nm/degree at 20 kg-com of applied torque.

Still a further object of the present invention is to provide a grid ormesh structure in accordance with any of the preceding objects in whichthe grid or mesh structure is an integral geogrid.

Yet another object of the present invention is to provide a method ofstrengthening a particulate material by embedding in the particulatematerial any of the above-described multi-axial grids or meshstructures.

Yet a further object of the present invention is to provide ageoengineering construction made up of a mass of particulate materialstrengthened by embedding therein a multi-axial geogrid or meshstructure in accordance with any of the above-described grids or meshstructures.

Still yet another object of the present invention is to provide a methodof retaining a particulate material by embedding any multi-axial grid ormesh structure as described above in the particulate material so thatthe particulate material is at least partially interlocked in theapertures or openings of the grid or mesh structure.

A final object to be set forth herein is to provide a method of making abiaxially oriented plastic material integral geogrid from a plasticssheet starting material by having holes arranged in an array ofsubstantially identical shape and size by selecting a starting materialthickness, a hole dimension, and pitch configuration of the array so asto produce oriented strands of the integral geogrid having an aspectratio greater than 1.0.

Definitions of Terms as Used Herein

The term “multi-axial” refers to a grid or mesh structure consisting ofstrands or ribs that are arranged along at least two different axeswithin the plane of the structure.

The term “geogrid” shall refer to a “multi-axial geogrid” unless theword “geogrid” is preceded by the word “uniax” or “uniaxial.”

The term “triangular geogrid” refers to a multi-axial geogrid havinggenerally triangular apertures or openings.

The term “rectangular geogrid” refers to a multi-axial geogrid havinggenerally rectangular apertures or openings.

The term “integral geogrid” shall refer to an oriented multi-axialgeogrid made by biaxially stretching a starting sheet material having adefined pattern of holes or depressions therein which form apertures inthe resulting multi-axial geogrid.

The terms “thick”, “thin”, “thickness”, “deep”, “depth”, and “shallow”refer to the dimension normal to the plane of the starting material ormesh structure and the terms “wide”, “narrow”, and “width” refer to theappropriate dimension in the plane of the starting material or meshstructure.

The term “aspect ratio” is defined as the most representative measure ofthe ratio of interconnecting rib thickness to width. Thickness and widthare typically measured at the mid-point of the rib, i.e. half-waybetween the junctions, providing that the rib dimensions are relativelyuniform. The regions where the ribs or strands intersect the nodes areexcluded from the measurements for determining aspect ratio. If the ribdimensions are not uniform, then the aspect ratio should be taken as thevalue that occurs most frequently along the rib's length betweenintersecting junctions, for example by constructing a histogram ofaspect ratio along the length of each set of parallel ribs in the gridstructure to determine the value of greatest frequency. Both thethickness and width of the ribs or strands and hence the aspect ratiovalues may vary along the length of the ribs or strands, especially asthey pass through connecting junctions or nodes.

The terms “aperture stability modulus”, ASM, or “torsional rigidity”refer to the relative stiffness of the geogrid apertures comprising ribsor strands and the junctions where they intersect in a multi-axialgeogrid as determined by test procedures referred to herein.

The term “vehicular traffic” refers to the loading that a civilengineering structure will experience as the result of the dynamic loadsthat occur from the passage of wheeled vehicles upon the surface of thestructure.

The term “oriented” means molecularly-oriented. In general, when anoriented strand is referred to, the direction of orientation is parallelto the longitudinal axis of the strand.

“Aperture” refers to the open areas in the geogrid structure, i.e. theopen areas that are bordered by the ribs or strands that can act toconstrain granular material.

“Uniax” and “biax” mean uniaxially-oriented and biaxially-oriented,respectively.

In relation to a mesh structure, “biaxially-oriented” means that themesh structure has been stretched in two directions generally at rightangles to each other.

The holes in the starting material may be through-holes or blind holes.If the holes are blind, the film or membrane in the hole will eitherrupture on stretching, or may remain as a thin membrane. The holes canbe formed by punching them out from the starting material, as disclosedin U.S. Pat. No. 4,374,798, or can be formed by extrusion as disclosedin U.S. Pat. No. 5,053,264, or can be formed by suitable embossing, orin any other appropriate manner.

“MD” is the machine direction, or in experimental work, the anticipatedmachine direction, normally the long dimension of the geogrid.

“TD” is the transverse direction, or in experimental work, theanticipated transverse direction, substantially at right angles to theMD.

The term “granular material” includes rocks, stones, gravel, sand,earth, clay, aggregate held by a binder such as asphalt or cement,concrete, or any other particulate or cohesive material used ingeotechnical engineering or building. The terms “soil” or aggregate asused herein have the same meaning as “particulate material”.

A “comparable conventional biax geogrid” is a conventional biax integralgeogrid having a low aspect ratio ribs (less than 1.0), made bystretching a plastics sheet starting material and stretching biaxially,the starting material being the same plastics material and the geogridhaving the same weight per unit area.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise” and the like are usedin an inclusive as opposed to an exclusive or exhaustive sense, that isto say, in the sense of “include, but not limited to”.

DESCRIPTION OF THE DRAWINGS

The objects of the invention, as well as many of the intended advantagesthereof, will become more readily apparent when reference is made to thefollowing description taken in conjunction with the accompanyingdrawings.

FIG. 1 is a plan view of a portion of a first starting material with theholes in a hexagonal pattern.

FIG. 2 corresponds with FIG. 1, but shows letters a, b and c for holespacing dimensions.

FIG. 3 is a plan view of the starting material shown in FIGS. 1 and 2which has been uniaxially stretched in the machine direction (MD).

FIG. 4 is a plan view of a multi-axial geogrid, having triangularapertures, made from stretching the material shown in FIG. 3 in thetransverse direction (TD).

FIG. 5 is a graph presenting results according to a traffic simulationtest that plots the rutting resistance of reinforced structurescontaining multi-axial geogrids having triangular apertures versus ribcross-sections having varying aspect ratios.

FIG. 6 is a plan view of a portion of a second starting material withthe holes in a rectangular pattern.

FIG. 7 is a plan view of the starting material shown in FIG. 6 afterhaving been uniaxially stretched in the machine direction.

FIG. 8 is a plan view of a multi-axial geogrid with rectangularapertures formed by stretching the material of FIG. 7 in the transversedirection.

DETAILED DESCRIPTION

Further scope of applicability of the present invention will becomeapparent from the detailed description and examples given hereinafter.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention willbecome apparent to those skilled in the art from this detaileddescription. Also, in describing the preferred embodiments, specificterminology as defined above will be resorted to for the sake ofclarity. It is to be understood that each specific term includes alltechnical equivalents which operate in a similar manner to accomplish asimilar purpose.

In a first preferred embodiment shown in FIGS. 1-4, a triangular geogrid10 shown in FIG. 4 is prepared from a starting material 1 shown inFIG. 1. The starting material is preferably a uniplanar sheet ofextruded plastics material having planar parallel faces, although othersheet-like materials can be used. Holes 2 are punched or formed in anarray of hexagons 3 of substantially identical shape and size so thatsubstantially each hole 2 is at a corner of each of three hexagons 3. Toproduce the triangular geogrid 10 from the punched sheet, the startingmaterial 1 is heated and a first stretch is applied in the notional MD,i.e., in a direction substantially parallel to the MD sides of thehexagons 3 shown in FIG. 1. The resulting uniaxially oriented grid 5,shown in FIG. 3, is then subsequently stretched in the TD to produce thebiaxially oriented triangular geogrid 10, shown in FIG. 4. The resultingmulti-axial geogrid 10 consists of triangular apertures 12 with ribs orstrands 14 that meet at each junction 16 with angles of approximately60°.

As shown in FIG. 4, the grid or mesh structure 10 includes a generallyuniform array of substantially straight oriented transverse strands orribs 18 interconnected in line by junctions 16 to extend transverselyacross the grid or mesh structure in spaced apart transversely extendingrows, generally designated by reference numeral 20. A plurality ofsubstantially straight oriented connecting strands or ribs 22interconnect the junctions 16 in adjacent rows 20, which together withthe transversely extending strands or ribs 18 form apertures or openings12 that have a generally equilateral triangular shape.

In accordance with the present invention, the thickness of the startingmaterial 1, and the dimensions for the spacing of the punched holes 2,noted as a, b, and c in FIG. 2, i.e., punched pitch, are selected sothat the aspect ratio of the ribs or strands 14 of the triangulargeogrid 10 is greater than 1.0, preferably in the range between about1.4 and about 2.2, but can vary as high as about 2.5, or above.

More specifically, if the hole spacing, i.e., punch pitch, is heldconstant, then the aspect ratio of the ribs or strands will increase asthe starting sheet thickness is increased. However, there is aninteraction effect between the starting punch pitch and the sheetthickness that determines the final rib aspect ratio of the finalgeogrid because both pitch and thickness can be varied independently.

In a second preferred embodiment shown in FIGS. 6-8, a rectangulargeogrid 30 shown in FIG. 8 is prepared from a starting material 32 shownin FIG. 6. As described in U.S. Pat. No. 4,374,798, the startingmaterial 32 shown in FIG. 6 is preferably a uniplanar sheet 36 ofextruded plastics material having planar parallel faces. However, otherextruded starting materials can be employed. Holes or depressions 34 arepunched or formed in a square or rectangular array 38 to produce themulti-axial geogrid 30 from the punched or formed starting sheet 32. Thestarting sheet 32 is heated and a first stretch is applied in thenotional MD, i.e., in a direction substantially parallel to the MD sidesof the rectangular hole pattern indicated in FIG. 6. The resultinguniaxially oriented geogrid 40, shown in FIG. 7, is subsequentlystretched in the TD to produce the biaxially oriented final product 30,as shown in FIG. 8. The resultant multi-axial geogrid 30 consists ofsquare or rectangular apertures 42 with ribs or strands 44 that meet ateach junction 46 with angles at approximately 90°.

As shown in FIG. 8, the grid or mesh structure 30 includes a generallyuniform array of substantially straight oriented transverse strands orribs 48 interconnected by junctions 46 extending transversely across thegrid or mesh structure in spaced apart transverse rows, generallydesignated by reference numeral 52. A plurality of substantiallystraight oriented connecting strands or ribs 54 interconnect thejunctions 46 in adjacent rows 52, which together with transverselyextending strands or ribs 50 form generally rectangular apertures oropenings 42.

In accordance with the present invention, the thickness of the startingsheet 32, and the size and spacing of holes or depressions 34, areselected so that the ribs or strands of the resultant rectangulargeogrid 30 have an aspect ratio greater than 1.0 and less than about4.0, with an accompanying aperture stability modulus (ASM) greater than0.3 Nm/degree at 20 kg-cm of applied torque and, more preferably,greater than 0.45 Nm/degree at 20 kg-cm of applied torque.

Test Methods for Examples

A general method for measuring the aperture stability modulus (ASM) forthe examples is outlined in “GRI Test Method GG9, Standard Test Methodfor Torsional Behavior of Bidirectional Geogrids when Subjected toIn-Plane Rotation,” Geosynthetic Research Institute, Mar. 10, 2004. Forthe ASM testing described herein, multi-axial geogrid samples havingapproximate dimensions of 350 mm×350 mm with a junction, or node,positioned exactly in the center of the frame were clamped all aroundtheir peripheries using a square clamping frame or containment box. Thetorquing device, consisting of a matched set of plates, was fastened tothe test sample using four bolts for conventional biaxial geogridproducts having strands or ribs intersecting at or nearly at 90 degreeangles.

In order to adapt the test method to the six-strand geogrid geometry,for example, the torquing device was modified such that the bolts wouldimmediately bear against the ribs or strands of the sample when thetorque was applied. In this case, a torquing device with six boltsspaced at 60 degrees around the device was employed. To carry out thetest, the torquing device was rotated relative to the perimeter clamp byapplying increasing amounts of torque in order to determine in-planetorsional rigidity, as described in test method referenced above withthe exception that only one loading cycle was performed. In the teachingof the '112 patent, results of the aperture stability modulus test werepresented as the number of degrees that the junction clamp attached tothe sample has rotated for the applied torque value of 4.5 Nm. Thesmaller the number of degrees of rotation for a given torque value, thehigher the ASM, or torsional rigidity, value. This convention is usedfor triangular geogrids in this specification. Another unit of measurefor reporting ASM test results for geogrids with rectangular aperturesis Nm/degree (Newton-meter per degree) at an applied torque value of 20kg-cm. The higher the Nm/degree value, the higher the torsional rigidityof the sample. For rectangular aperture geogrids in this specification,AMS values are reported using Nm/degree at 20 kg-cm applied torque.

The performance of a multi-axial geogrid for resisting rutting due tovehicle traffic was evaluated using a new small-scale test to simulatewell-established field tests such as the one described by Webster(above). The small-scale test is designed to reproduce the results ofwell-established field tests for traffic performance of multi-axialgeogrids and comprises a test section consisting of an underlying claysubgrade, a single layer of geogrid, and a compacted granular sub base.The test section is subjected to the load of a single weighted wheel.The wheel traverses the test section along a single horizontal path,constantly reversing direction from one end of the test section to theother end. A control test section with no geogrid present will rapidlyfail under such testing. For example, after 1000 passes of the wheel onan unreinforced test section, a deep rut will be formed. By usingproperly designed multi-axial geogrids as reinforcement, decreasedamounts of rutting depth will occur for a given number of wheel passescompared to an unreinforced test section. This decreased rut depth hasan impact on the lifetime of the civil engineering structure and canextend this lifetime by factors of up to 50 times that of anunreinforced structure. Hence, a roadway or other civil engineeringstructure reinforced in accordance with the present invention will haveincreased longevity and decreased maintenance requirements.

EXAMPLES

FIGS. 1 to 5 and Table 1—First High Aspect Ratio Samples

In a first set of high aspect ratio rib samples configured according tothe present invention, the samples were prepared as described inaccordance with the FIGS. 1-4 embodiment using the preferred strictlyuniform starting material. The dimensions for the spacing of the punchedholes, or pitch, noted as a, b, and c in FIG. 2, was varied. In thesesamples, the resulting multi-axial geogrid consisted of triangularapertures with ribs or strands that meet at each junction with angles atapproximately 60°.

TABLE 1 First Set of Geogrid Samples According to the Present Inventionwith Triangular Apertures Sheet thickness, Dimension Dimension DimensionRib Aspect Example mm a, mm b, mm c, mm Ratio C1 4.7 9.5 10.5 4 0.63 C24.7 10.63 11.52 4.43 0.38 1 3.2 6.19 6.71 2.58 1.06* 2 3.4 6.19 6.712.58 0.97 3 3.4 5.41 5.86 2.26 1.02 4 3.4 4.64 5.03 1.94 1.19 5 3.4 3.864.19 1.61 1.88 6 3.6 6.19 6.71 2.58 1.19* 7 3.8 6.19 6.71 2.58 1.2 8 46.19 6.71 2.58 1.26 9 4 5.41 5.86 2.26 1.39 10 4 4.64 5.03 1.94 1.56 114 3.86 4.19 1.61 2.19 12 4.8 7.74 8.35 3.22 1.27* 13 4.8 6.19 6.71 2.581.4 14 4.8 5.41 5.86 2.26 1.81* 15 4.8 4.64 5.03 1.94 2.1* 16 4.8 3.864.19 1.61 2.55* 17 5.8 7.74 8.35 3.22 1.53* 18 5.8 6.19 6.71 2.58 2.0119 5.8 5.41 5.86 2.26 2.18* 20 5.8 4.64 5.03 1.94 2.54* 21 5.8 3.86 4.191.61 3.08* 22 6.8 6.19 6.71 2.58 2.2 *Predicted

Table 1 presents geogrid Samples 1 through 22 to illustrate the instantinvention using triangular apertures (a few of the samples are fromactual tests, the others are representative), along with ComparativeExamples C1 and C2 taken from data presented in the '112 patent.Compared to the '112 patent, the spacing or pitches of the holes, shownas dimensions a, b, and c in FIG. 2, have been reduced for the instantinvention in order to produce the higher aspect ratio rib shape. Asshown in Table 1, it is possible to obtain a wide range of rib aspectratio values greater than unity by varying both punch pitch and startingsheet thickness. For example, using a small punch pitch, i.e. close holespacing, the aspect ratio of the ribs can be significantly higher thanfor the Comparative Examples even when the starting sheet thickness isless than that of the Comparative Examples.

In the '112 patent, a key objective was to obtain a high value ofaperture stability modulus compared to previously established commercialproducts based on Webster's findings. The aperture stability modulus forComparative Example C2, as taken from FIG. 13 of the '112 patent, is 6.7degrees of rotation at 4.5 Nm torque. The smaller the number of degreesof rotation for the specified 4.5 Nm torque value, the higher the ASMvalue. The '112 patent indicates that ASM was increased 65% relative toa comparable conventional biax geogrid tested under the same testconditions. (See FIG. 13 of the '112 patent and related description inthe specification.) At the time it was believed that this increase in ageogrid's ASM would be favorable for improving the resistance of areinforced structure to rutting by vehicular traffic.

According to the instant invention, however, an objective is to increasethe triangular geogrid's rib aspect ratio, rather than maximizing ASM,in order to improve resistance to rutting. It has been observed that ASMhas in fact decreased for samples according to the present inventioncompared to the test samples of the '112 patent, i.e. triangular geogridsamples tested for the instant invention have ASM values between 16 and21 degrees of rotation at 4.5 Nm torque. The rutting resistance of areinforced structure has, however, substantially improved compared to areinforced structure according to the '112 patent, despite thesignificantly decreased ASM. Even though ASM values for samplesaccording to the present invention are lower than for the '112 patentexamples, the ASM values are nevertheless indicative of a stiffmulti-axial geogrid with rigid junctions. The combination of anadequately rigid geogrid aperture plus the high aspect ratio rib shapeproduces superior performance, i.e. rutting resistance, in thereinforced structure. Furthermore, these first samples combine theaforementioned rigidity and high aspect ratio rib with the advantage ofimproved load distribution demonstrated in the '112 patent arising fromthe geometrical arrangement of six ribs attached to each junction at 60°angles and triangular apertures.

FIG. 5 displays in graphic form the rutting resistance of reinforcedstructures containing multi-axial geogrids having triangular aperturesas described herein versus rib cross-sections of varying aspect ratios.FIG. 5 presents the results according to a traffic simulation test thatwas carried out as described under “Test Methods” above. The resultsdemonstrate that resistance to rutting improves substantially as theaspect ratio of the geogrid rib is increased. FIG. 5 comparesintegral-junction geogrids having triangular apertures as describedwhich possess rib aspect ratios ranging from 0.38 to 2.2. The low aspectratio sample, corresponding to Comparative Example C2, was producedusing the teaching from the '112 patent, and the samples with aspectratios greater than unity are according to the instant invention.

As demonstrated by the examples of Table 1, rib aspect ratio can beincreased as desired by employing even thicker plastics sheet for thestarting material or by further modifying the punching conditions suchas the hole sizes, shapes, and spacing, or by other techniques thatcould be developed by those skilled in the art.

The types of starting materials for the plastics sheet, the nature ofthe holes or depressions used to form the finished products, theavailable methods of manufacture, and other desired features for thefinal geogrid or mesh structure have been described in the prior art,including the '112 patent and other patents cited hereinbefore, andfurther explanation is not deemed necessary for those skilled in theart.

FIGS. 6 to 9 and Table 2—Second High Aspect Ratio Samples

In a second set of high aspect ratio rib samples configured according tothe present invention, the starting material 11 shown in FIG. 6 was astrictly uniplanar sheet of extruded plastics material having planarparallel faces. Holes or depressions 12 are punched to form a square orrectangular array. To produce the multi-axial geogrid product from thepunched sheet, the starting material 11 was heated and biaxiallystretched as described above. In these samples, the resultingmulti-axial geogrid consists of square or rectangular apertures withribs or strands that meet at each junction with angles at approximately90°.

TABLE 2 Second Set of Geogrid Samples According to the Present Inventionwith Rectangular Apertures Actual (measured) Predicted surface TrafficImprovement Sheet Rib Aperture stability surface deformation deformationat Factor (TIF) versus Exam- thickness aspect modulus, Nm/⁰ at at 10,000passes 10,000 passes unreinforced test ple (mm) ratio 20 kg-cm torque(mm) (mm) section C3 6.8 0.76 0.30 C4 3.1 0.34 0.38 57 49.7 3.32 23 40.52 0.27 52.4 53.1 2.44 24 4.8 0.57 0.36 46.2 50.1 4.57 25 4 0.86 0.2552.1 52.9 1.99 26 6.8 1.22 0.50 39.2 44.3 23.5 27 7.5 1.92 0.50 46.243.3 3.27 28 7.5 3.68 0.38 44.2 43.8 5.78

The above Table 2 presents geogrid samples 23 through 28 to illustratethe instant invention using rectangular apertures. Comparative ExampleC3 is a biaxial geogrid with square apertures sold commercially asTensar type SS-30, and C4 is a similarly produced commercial productwith rectangular apertures sold as Tensar BX1100. Samples 23 through 25are additional comparative examples with AR less than 1.0 that areincluded for reference. Samples 26 through 28 were produced according tothe instant invention with a high aspect ratio rib cross-section. Inorder to increase the rib aspect ratio for Samples 26 through 28, thestarting sheet thickness, the punched hole size and the hole spacingwere varied in a manner similar to that described for Samples 1 through22 of Table 1. As shown, samples 26, 27 and 28 illustrate the ability toachieve rib aspect ratios greater than unity by manipulation of sheetthickness, punch pitch, and hole size.

Table 2 indicates that the best performance, i.e. the minimum rut depthvalue of 39.2 mm, occurs at a rib aspect ratio of 1.22 for the limitednumber of samples produced. The expected improvement in performance,i.e. rutting resistance, for samples with rib aspect ratios greater than1.0 is demonstrated. Table 2 also shows the “Traffic ImprovementFactor,” defined as ratio of the time to reach a specified rut depth fora test sample relative to the time to reach the same rut depth with nogeogrid reinforcement present. Note that Sample 26 with a 1.22 ribaspect ratio has a Traffic Improvement Factor (TIF) of 23.5, i.e. 23.5times the lifetime compared to an unreinforced soil. Samples 26 through28 generally have rut depths that are significantly lower thancomparative example C4 and examples 23 through 25. The mean rut depth is51.9 mm for the four samples with rib aspect ratio less than one, i.e.C4 and samples 23 through 25. The mean rut depth for samples 26 through28, with rib aspect ratio greater than one, is 43.2 mm. The meanreduction in rut depth for the instant invention (rib aspect ratiogreater than 1.0) compared to samples with rib aspect ratio less than1.0 is 17%. Looking at the Traffic Improvement Factor, the mean TIFincreases from 3.08 for samples with an aspect ratio less than 1.0 to amean TIF of 14.6 for samples with an aspect ratio greater than 1.0. Thelongevity of the civil engineering structure in terms of trafficimprovement factor for the instant invention is thus shown to beimproved.

One observes that Sample 28, although possessing the highest rib aspectratio, does not exhibit the best performance as measured by rut depth orTIF. Further investigation was made, and the aperture stability modulus(ASM) was also considered. Table 2 indicates that Sample 28 has arelatively low ASM value such that the benefit of the high rib aspectratio has been offset somewhat by the relatively low ASM value.

A multi-linear model was constructed to examine the impact of both ribaspect ratio and ASM for rectangular geogrids. For the examples in Table2, the following model was generated by performing a least-squaresregression:Rut depth at 10,000 passes=62.4−1.83*Rib Aspect ratio−31.4*ApertureStability Modulus (Nm/degree at 20 kg-cm applied torque).Therefore, the rut depth in rectangular geogrids is observed to beimpacted by the combination of two geogrid properties, i.e. rib aspectratio and ASM. As explained in the background of the invention, this isconsistent with a known correlation between rectangular-aperture geogridperformance and ASM. As seen from the examples in Table 2 and inaccordance with the numerical model, one can vary both aspect ratio andaperture stability modulus to arrive at an optimum product performance.For rectangular geogrids, the preferred ASM is greater than 0.3Nm/degree at 20 kg-cm applied torque and more preferably greater than0.45 Nm/degree at 20 kg-cm applied torque.

High aspect ratio rib geogrids made by the methods described in both thefirst and second sets of samples, as outlined above, can be made with awide range of thicknesses for the starting sheet from about 3.0 mm to atleast about 9.0 mm.

Polymeric grids and meshes have also been used in various commercial andgeotechnical applications such as fencing (U.S. Pat. No. 5,409,196),cellular confinement (U.S. Pat. No. 5,320,455), mine stopping (U.S. Pat.No. 5,934,990) and other commercial enclosure, containment and barrierapplications. The present invention can have certain advantages overknown products for these applications. For example, in mine stopping,sealant, such as shotcrete, is sprayed onto the mesh structure toprevent air flow. The problem with the lower aspect ratio grids is thatthe sealant material tends to rebound off the wider rib surface and thusdoes not adhere as well and/or more sealant is required. With a higheraspect ratio product as in the present invention, the spray-on materialshould adhere more readily and a lesser quantity is thus required toachieve the desired barrier effect.

Alternate Embodiments

Following the teaching from this invention, other methods formanufacturing multi-axial geogrids with high aspect ratio ribs can besimilarly demonstrated by relatively simple modifications to theexisting methods of manufacturing geogrids, for example by stitchbonding fabrics made of, for instance, polyester filaments and applyinga flexible coating such as a PVC coating, or by weaving or by knitting,by spot-welding oriented plastic strands together, by extruding undrawnparallel filaments into a net structure and subsequently stretching thestructure, or by other methods of multi-axial geogrid manufacture knownto those skilled in the art. One need only apply the principle ofincreasing the aspect ratio of the rib dimensions as taught by thisinvention. Such multi-axial geogrids can have rectangular aperturesconsisting of longitudinal and transverse strands or ribs, or thestrands can be arranged to meet at the junctions with angles not equalto 90°. Stiff junctions are preferred as a desirable, but not a solecondition, to contribute toward minimizing the rutting effects ofvehicular traffic.

The invention being thus described, it will be apparent that the samemay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be recognized by one skilled in the art areintended to be included within the scope of the following claims.

What is claimed is:
 1. A multi-axial geogrid comprising a firstgenerally uniform array of substantially straight highly orientedstrands or ribs extending in a first direction and interconnected byintegrally-formed junctions extending across said geogrid in spacedapart rows and a second generally uniform array of substantiallystraight highly oriented connecting strands or ribs extending in asecond direction and interconnecting said integrally-formed junctions inadjacent rows to form apertures or openings between adjacent orientedstrands or ribs and junctions, said multi-axial geogrid being formed bybiaxially stretching a polymer starting material having a generally flatsheet configuration with holes or depressions which form saidmulti-axial geogrid apertures or openings when biaxially stretched, saidstrands or ribs in said multi-axial geogrid that extend in the firstdirection having an aspect ratio greater than 1.0 after being stretched.2. The multi-axial geogrid of claim 1, wherein the geogrid hastriangular apertures or openings, and said aspect ratio is betweengreater than 1.0 and about 2.5.
 3. The multi-axial geogrid of claim 1,wherein said polymer starting sheet material is substantially uniplanar.4. The multi-axial geogrid of claim 1, wherein the geogrid hasrectangular apertures or openings, and said aspect ratio is in the rangebetween greater than 1.0 and about 4.0.
 5. The multi-axial geogrid ofclaim 1, wherein the connecting strands or ribs that extend in thesecond direction have an aspect ratio greater than 1.0 after beingstretched.
 6. The multi-axial geogrid of claim 1, wherein said geogridis embedded in a particulate material so that the particulate materialis at least partially interlocked in said apertures or openings.
 7. Themulti-axial geogrid of claim 1, wherein the geogrid includes a series ofparallel strands or ribs intersected at integrally-formed junctions witha second series of parallel strands or ribs that are essentiallyperpendicular to the first series of parallel strands or ribs to formgenerally rectangular apertures or openings.
 8. The multi-axial geogridof claim 1 wherein the geogrid has three or more series of parallelstrands or ribs intersecting each other at integrally-formed junctionssuch that the angles of the intersecting strands or ribs at the junctionare angles not equal to 90°.
 9. The multi-axial geogrid of claim 8wherein the geogrid has three series of parallel strands or ribsintersecting each other at the integrally-formed junctions at angles ofabout 60° to form generally equilateral triangular apertures oropenings.
 10. The multi-axial geogrid of claim 1 wherein the strands orribs have a concave cross-section at or near their center.
 11. A civilengineering reinforcing structure, comprising a multi-axial geogridformed by biaxially stretching a generally flat polymer starting sheethaving a plurality of spaced holes therein to generate a generallyuniform array of substantially straight molecularly oriented strands orribs interconnected by partially oriented junctions extending acrosssaid geogrid in spaced apart rows in a first direction and a generallyuniform array of substantially straight molecularly oriented connectingstrands or ribs extending in a second direction and interconnecting saidpartially oriented junctions in adjacent rows to form apertures oropenings between adjacent molecularly oriented strands or ribs andpartially oriented junctions, said molecularly oriented strands or ribsof said multi-axial geogrid in the first direction having an aspectratio at a longitudinal midpoint which is greater than 1.0 after beingstretched, said aspect ratio being achieved by manipulation of startingsheet thickness, hole spacing and hole size.
 12. The geogrid of claim11, wherein said geogrid is embedded in a particulate material so thatthe particulate material is at least partially interlocked in saidapertures or openings.
 13. The geogrid of claim 11, wherein the strandsor ribs in the array extending in the first direction are essentiallyperpendicular to the strands or ribs in the array extending in thesecond direction to form generally rectangular apertures or openings.14. The geogrid of claim 13, wherein the aspect ratio is in the rangebetween greater than 1.0 and about 4.0.
 15. The geogrid of claim 11,wherein the geogrid has a third array of oriented strands or ribsintersecting each other at integrally-formed junctions such that theangles of the intersecting strands or ribs of the three arrays at thejunctions are angles not equal to 90°.
 16. The geogrid of claim 11,wherein the geogrid has rectangular apertures or openings.
 17. Thegeogrid of claim 11, wherein the geogrid has triangular apertures oropenings, and said aspect ratio is between greater than 1.0 and about2.5.
 18. The geogrid of claim 11, wherein said polymer starting sheetmaterial is substantially uniplanar.
 19. The geogrid of claim 11 whereinthe strands or ribs have a concave cross-section at or near theircenter.
 20. The geogrid of claim 11 wherein said molecularly orientedstrands or ribs in the second direction have an aspect ratio at alongitudinal midpoint of said molecularly oriented strands or ribs whichis greater than 1.0 after being stretched.